The 3rd Generation Partnership Project (3GPP), which undertakes activities for standardizing recent mobile communication systems, is in the process of creating specifications for the Evolved Packet System (EPS), which realizes an all-IP architecture. In 3GPP, an access system connected to EPS is under study while taking into consideration not only a case of LTE but also a case of wireless LAN.
Furthermore, in 3GPP, a Proximity based Service (ProSe), as described in NPL 1, has been studied in the process of creating the specifications for EPS. ProSe is aimed at providing a service in which a function for User Equipment (UE) as a communication terminal to detect another proximity communication terminal (discovery), a function to establish direct communication between pieces of UE without passing through a core network, a base station or the like (direct communication), and the like are given.
In ProSe, since direct communication can be made between terminals without passing through a base station, a core network to which an access network is connected, or the like, concentration on the access network, the core network, or the like can be prevented (congestion prevention) so that an offloading effect can be expected.
Furthermore, in ProSe, in order for a terminal outside a communication range of a base station to access the network, a UE-to-NW Relay in which a terminal inside the communication range becomes a relay terminal for the terminal outside the communication range is also under study now.
To be specific, the UE-to-NW Relay is a service achieved by the terminal inside the communication range and the terminal outside the communication range of the base station establishing a direct communication path.
Note that, a terminal device having a function of connecting to a PDN connection to the network, even from the outside of the communication range, is referred to as Remote UE, and a terminal device configured to provide the PDN connection to the Remote UE by a relay is referred to as Relay UE.
To establish a direct communication path between proximity terminal devices, ProSe needs a service in which a terminal device seeks and detects a terminal device as a communication target in the direct communication. To rephrase, a function for a terminal device to detect a proximity terminal device (discovery) is needed. In ProSe, as an example of this type of detection method, a detection method in which Relay UE transmits an announcing signal to Remote UE to notify the Remote UE of that the Relay UE has a Relay function while the Remote UE monitors the announcing signal has been studied. This type of detection method is defined as a “Model A”.
Further in ProSe, as an example of a detection method, a detection method in which the Remote UE transmits a Solicitation signal to the Relay UE and the Relay UE having received the Solicitation message from the Remote UE replies a Response whereby the Remote UE detects the Relay UE has been studied. This type of detection method is defined as a “Model B”. Note that the Solicitation signal may be an announcing signal and the Relay UE may receive the announcing signal from the Remote UE by monitoring.
Moreover, as a direct communication path between pieces of UE, usage of two schemes are being considered in ProSe. The first one is a method using an LTE access technique. The second one is a method using a Wireless LAN (WLAN) access technique.
Furthermore, non-Public Safety and Public Safety are defined in ProSe. The non-Public Safety assumes a commercial service by a mobile network operator, and is available only in a case of UE being positioned within a communication range of an LTE base station. On the other hand, the Public Safety assumes usage by a disaster prevention radio system, and is available not only in a case of UE being positioned within a communication range of the LTE base station but also a case of UE not positioned within the communication range of the LTE base station (eNB).